Regular physical activity has been implicated in the prevention of a number of cancers including those of the colon, breast, endometrium, and prostate (Kaaks & Lukanova, 2002), and is potentially associated with the prevention of other cancers as well (Friedenreich, 2001). Despite the benefit of regular physical activity in the prevention of cancer as well as other debilitating illnesses, 79% of the U.S. population do not get the recommended amount of physical activity as defined by 30 minutes of regular activity 5 or more days per week, and 40% of the population is completely sedentary (U.S. Department of Health and Human Services (USDHHS), 1998). The long-term objective of the proposed research is to advance a conceptualization of the biological, physiological, and psychological determinants of exercise behavior with the express purpose of developing more effective prevention and intervention efforts that are designed to promote exercise behavior among sedentary individuals, and to prevent cancers associated with sedentary lifestyle. The goal of the present application is to establish readily identifiable phenotypic markers for the reinforcing effects of exercise by rigorously testing theory-based hypotheses regarding individual differences in the effects of exercise on mood states. Using a laboratory study and a longitudinal follow-up, the proposed study will address three hypotheses: 1) That a single bout of cardiovascular exercise as compared to a bout of sedentary activity will increase positive mood and decrease negative mood, 2) That body temperature and/or cortisol release moderate the effects of acute exercise on positive and negative mood, and 3) That more positive mood changes in the laboratory predict actual exercise behavior prospectively. We expect that greater increases in positive mood/decreases in negative mood--potentially moderated by body temperature or cortisol--will be associated with a higher level of exercise behavior. Once we identify endophenotypes that are narrowly defined, associated with the broad phenotype (i.e., regular exercise behavior), and related to an underlying biological mechanism, we can use these endophenotypes in future research to search for and identify specific genetic mechanisms underlying individual differences in voluntary exercise motivation and behavior. The ultimate goal of this research is to develop exercise promotion interventions for sedentary individuals that incorporate knowledge about individual differences in the experience of immediate reward after exercise.